[datlechin]

Why

With a dead-key layout (U.S. International, Brazilian), ' then Space typed ''' in the SQL editor, and " typed """. Every composition keystroke ran through the typing filters: the commit replaced the marked ' with ', and the pair filter read a quote replacing a range as "wrap the selection". Each composition stage also went into the undo history.

Change

• Dead key then Space types one quote, and the next key pairs it as on a US layout.
• One undo removes a committed accent or input method word, with no stages in between.
• Double, triple or Shift-click during a composition keeps the composed text, as NSTextView does.
• Cancelling a dead key typed before a closing quote keeps that quote.

Verified

CompositionTypingTests and CompositionUndoTests fail on main and pass here. The undo, click and unmark behavior matches NSTextView, measured with a probe.

Not in this PR

• A composition typed over a selection takes two undos to restore it; NSTextView takes one.
• Touching selections still merge into one caret after a composition commit, as on main.

Fixes #3458